This invention relates to circuits for electronic strobe lights such as are used to provide visual warning of potential hazards or to draw attention to an event or activity. Such devices are used in electronic fire alarm systems and are frequently associated with audible warning devices such as horns, and provide an additional means for alerting those persons who may be in danger. Strobe alarm circuits include a flashtube and a trigger circuit for initiating firing of the flashtube, with energy for the flash typically supplied from a capacitor connected in shunt with the flashtube. In some known systems, the flash occurs when the voltage across the flash unit (i.e., the flashtube and associated trigger circuit) exceeds the threshold value required to actuate the trigger circuit, and in others the flash is triggered by a timing circuit. After the flashtube is triggered it becomes conductive and rapidly drains the stored energy from the shunt capacitor until the voltage across the flashtube has decreased to a value at which the flashtube extinguishes and becomes non-conductive. In a more specific sense, the present invention relates to apparatus for charging the energy-storing capacitor.
Typical of prior art strobe light devices is that described in U.S. Pat. No. 4,952,906 entitled "Strobe Alarm Circuit" for flashing a flash unit at a desired frequency. The capacitor connected in shunt with the flash unit is charged from an inductor which is repetitively connected and disconnected across a D.C. power line by a switch so that energy is stored in the inductor during the period when the switch is closed and transferred to the capacitor when the switch is open. During the open period of the switch the parallel combination of the flash unit and the capacitor is connected across the inductor in such a manner that current will not flow from the power line through the flash unit or the capacitor. The switch is repetitively cycled between open and closed, the open period being initiated in response to the current flowing through a resistor connected in series with the inductor attaining a particular value and the closed period being initiated in response to a periodic timing signal. When the switch has been cycled the number of times required to transfer sufficient energy to the capacitor to attain the threshold firing voltage of the flash unit, the capacitor discharges its stored energy through the flashtube.
This circuit has the disadvantage that power is needlessly wasted in the resistor connected in series with the inductor for determining when the current flowing though the inductor has attained a particular value.
Although the strobe flashing rate of the circuit of the '906 patent is said to be determined independently of the supply voltage, because firing of the flashtube depends on the voltage of the storage capacitor reaching a firing threshold voltage, both the flash rate and flash intensity will vary with changes in capacitance of the storage capacitor. As the electrolytic capacitor normally used as the storage capacitor has a wide capacitance tolerance, the flash rate and intensity of the flash could vary greatly from unit to unit. Additionally, the light output and flash rate will be directly affected by changes in capacitance due to aging and variations in operating temperatures.
In order to overcome the described disadvantages and shortcomings of known prior art circuits, the present invention has as an object to provide an improved strobe light circuit the flash rate of which is not dependent o the supply voltage.
Another object of the invention is to provide a strobe light circuit which produces a substantially constant light output independent of the capacitance tolerance of the energy storage capacitor.
A further object is to provide a strobe light circuit which has a higher operating efficiency than prior art circuits by avoiding the power loss in a resistor used in the prior circuit to sense charging current.